Field of the Invention
The Present Invention Relates to Mobile Communication.
Related Art
3rd generation partnership project (3GPP) long term evolution (LTE) evolved from a universal mobile telecommunications system (UMTS) is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) evolved from the 3GPP LTE.
As set forth in 3GPP TS 36.211 V10.4.0, the physical channels in 3GPP LTE may be classified into data channels such as PDSCH (physical downlink shared channel) and PUSCH (physical uplink shared channel) and control channels such as PDCCH (physical downlink control channel), PCFICH (physical control format indicator channel), PHICH (physical hybrid-ARQ indicator channel) and PUCCH (physical uplink control channel).
In a next-generation mobile communication system, it is expected that a small cell having a small cell coverage radius will be added to the coverage of an existing cell and a small cell will process more traffic.
However, if small cells within the coverage of a macro cell are densely deployed, it may be difficult for UE to detect the small cells within a short time.
In order to solve this problem, small scale cells may transmit a new discovery signal (DS) in addition to the existing PSS/SSS. Hence, the UE may identify small scale cells by using the discovery signal.
Further, as more communication devices require large communication capacity, efficient utilization of the restricted frequency band in the next generation is becoming a more important requirement.
A cellular communication system such as an LTE system is considering a method of utilizing an unlicensed band such as 2.4 GHz band which used by the existing IEEE 802.11 system, that is, the wireless local area network (WLAN) and an unlicensed band such as 5 GHz which is newly drawing attention, in the traffic offloading.
Basically, the unlicensed band assumes the scheme of wirelessly transmission and reception through the competition between respective communication nodes, and thus it requires confirming whether another communication has not transmitted signals by performing channel sensing before each communication node transmits signals. This is called a clear channel assessment (CCA), and the eNodeB or user equipment (UE) of the LTE system may also need to perform CCA for signal transmission in the unlicensed band.
However, the neighboring node of IEEE 802.11 system transmits signals, and thus when the CCA is not confirmed, the small scale cell cannot transmit the discovery signal. In other words, the small scale cell cannot transmit the discovery signal if it is determined that the signal size of the neighboring node based on IEEE 802.11 system is a threshold or greater.
As such, if the discovery signal cannot be periodically transmitted and is only intermittently transmitted (e.g., the discovery signal may be absent during tens of continuous periods), the UE cannot quickly identify the small scale cell.